Trying to understand coolant expansion tanks (classic Saab 900)

So I'm doing the subaru SVX swap into my impreza, which has lead me to use a Saab 900 turbo radiator. The radiator does not have a 'typical' spring loaded fill cap with a coolant recovery/catch/overflow tank. It has a 1/4" nipple on the top of one end tank which goes to a coolant expansion tank with a pressure cap, which then gets returned directly to the water pump via the bottom of the tank.

I haven't really been able to find any info that actually explains how this system works. I don't see how it can regulate water pressure in a closed system. Does it just bleed off air?

And finally, is there any way to regulate cooling system pressure through that 1/4" nipple? Hmm, nipple.

(A thought I had was to run a line to the bottom of a catch can with a regular radiator cap on top, which bled off air instead of water, but I don't know if this method would work at all... and it would make filling the radiator a bit of a joke)

The remote header tank system you describe is quite common, especially in racing applications. A small part of the coolant circulates through the tank, taking any air that makes its way to the radiator with it. The return line, being at the bottom of the tank, draws only coolant back into the system. The whole business is under the pressure of the cap. If a non-opaque plastic tank is used, coolant level is easily checked.

It actually is an extension of the radiator.

On the Classic 900. The line that feeds the expansion chamber is teed off of the line that feeds the remote waterpump. (as the saab engine is backwards, it has an external waterpump to ease maintance) this may make plumbing in the resevour difficult into a non-saab.

You are correct in thinking that the resevour is under constant pressure from the cooling system and that the small "airline" does indeed return to the radiator. I will take a closer look at my resevour tomarrow and let you know what it does with overflow

Chas_H wrote: The remote header tank system you describe is quite common, especially in racing applications. A small part of the coolant circulates through the tank, taking any air that makes its way to the radiator with it. The return line, being at the bottom of the tank, draws only coolant back into the system. The whole business is under the pressure of the cap. If a non-opaque plastic tank is used, coolant level is easily checked.

Yes but how does it maintain a certain pressure? Does it relieve pressure by bleeding air or water? Where does it go?

mad_machine wrote: On the Classic 900. The line that feeds the expansion chamber is teed off of the line that feeds the remote waterpump. (as the saab engine is backwards, it has an external waterpump to ease maintance) this may make plumbing in the resevour difficult into a non-saab. You are correct in thinking that the resevour is under constant pressure from the cooling system and that the small "airline" does indeed return to the radiator. I will take a closer look at my resevour tomarrow and let you know what it does with overflow

So that line is designed to move air, not allow movement of coolant to facilitate pressure changes?

Taiden wrote:

Chas_H wrote: The remote header tank system you describe is quite common, especially in racing applications. A small part of the coolant circulates through the tank, taking any air that makes its way to the radiator with it. The return line, being at the bottom of the tank, draws only coolant back into the system. The whole business is under the pressure of the cap. If a non-opaque plastic tank is used, coolant level is easily checked.

Yes but how does it maintain a certain pressure? Does it relieve pressure by bleeding air or water? Where does it go?

The pressure cap maintains pressure, just like any other cooling system. If the system exceedes the limits of the cap, the cap opens and bleeds off whatever is directly under the cap. If there is coolant directly under the cap that's what gets bled out. Usually there is a hose to direct whatever is bled off to the ground. It's possible that hose is missing in your case.

Chas_H wrote:

Taiden wrote:

Chas_H wrote: The remote header tank system you describe is quite common, especially in racing applications. A small part of the coolant circulates through the tank, taking any air that makes its way to the radiator with it. The return line, being at the bottom of the tank, draws only coolant back into the system. The whole business is under the pressure of the cap. If a non-opaque plastic tank is used, coolant level is easily checked.

Yes but how does it maintain a certain pressure? Does it relieve pressure by bleeding air or water? Where does it go?

The pressure cap maintains pressure, just like any other cooling system. If the system exceedes the limits of the cap, the cap opens and bleeds off whatever is directly under the cap. If there is coolant directly under the cap that's what gets bled out. Usually there is a hose to direct whatever is bled off to the ground. It's possible that hose is missing in your case.

So it would then seem that it bleeds air to maintain a specific pressure in the cooling system.

The pressure cap is on the expansion tank, not the radiator. There is generally no unpressurized container, as the expansion tank is filled only half way, allowing room for expansion. Hence the term "expansion tank". You need a fitting somewhere in the cooling sustem to connect the lower, larger line back into the rad, lower rad hose or water pump intake. The small line at the top of the rad is just to bleed fluid or air off the top of the rad to keep the rad completly full. The expansion tank should also be mounted fairly high.

Failing that, buy one of these : http://www.summitracing.com/parts/CTR-80-200/ and plug the rad vent hose into a fitting on the bottom of this tank, and plug the other fittings in the pressurized portion. It will be slow to fill, since you are filling the cooling system through a 1/4 inch hose. It will also need to be held above the rad when filling. The spigot on the cap fitting needs to be vented.

Third option is to find a rad cap fitting that clamps into the upper rad hose and block the 1/4 inch fitting on the rad. You can then add a traditional vented catchcan. IIRC, 4.7 Dakotas had the rad cap on the upper rad hose.

So it would then seem that it bleeds air to maintain a specific pressure in the cooling system.

If the system pressure excedes the rating of the cap, it will release what ever is directly under it to reduce pressure. If it's air under the cap, that's what comes out. If it's coolant under the cap, that's what comes out. Just like the cap on a radiator.

pretty much every new vehicle built in about the last 20 years has this setup.. just think of the expansion tank as the highest part of the radiator, and there is no overflow tank.

novaderrik wrote: pretty much every new vehicle built in about the last 20 years has this setup.. just think of the expansion tank as the highest part of the radiator, and there is no overflow tank.

Seriously? I'm no expert on newer vehicles, but the Saabs and VWs are the only cars I have ever seen with an expansion tank. All other cars I have seen with the hood up have spring-loaded pressure caps in a filler neck on the end tanks of the radiator which vents coolant to an overflow bottle.

Streetwiseguy wrote: The pressure cap is on the expansion tank, not the radiator. There is generally no unpressurized container, as the expansion tank is filled only half way, allowing room for expansion. Hence the term "expansion tank". You need a fitting somewhere in the cooling sustem to connect the lower, larger line back into the rad, lower rad hose or water pump intake. The small line at the top of the rad is just to bleed fluid or air off the top of the rad to keep the rad completly full. The expansion tank should also be mounted fairly high. Failing that, buy one of these : http://www.summitracing.com/parts/CTR-80-200/ and plug the rad vent hose into a fitting on the bottom of this tank, and plug the other fittings in the pressurized portion. It will be slow to fill, since you are filling the cooling system through a 1/4 inch hose. It will also need to be held above the rad when filling. The spigot on the cap fitting needs to be vented. Third option is to find a rad cap fitting that clamps into the upper rad hose and block the 1/4 inch fitting on the rad. You can then add a traditional vented catchcan. IIRC, 4.7 Dakotas had the rad cap on the upper rad hose.

Thanks streetwise. I actually (randomly) ran into the Dakota/Durango filler neck. I want to use this setup but there's little room for a filler neck in the upper radiator hose. I think I may have to make it work though.

An acquaintance told me that he had to use a 28 psi cap in his external filler neck, because anything below would just blow out water with every pulse of the water pump. Do you have any idea why this would happen? It seems so bizarre to me. The stock cap in that car was 14 psi if I recall correctly.

In reply to Taiden:

Most GMs have had expansion tanks for a long time as well. The radiators in most modern cars are a bit low to clear the hood. The tank alows them to move the cap back to the highest point so it can be filled and bleed easily.

Wally wrote: In reply to Taiden: Most GMs have had expansion tanks for a long time as well. The radiators in most modern cars are a bit low to clear the hood. The tank alows them to move the cap back to the highest point so it can be filled and bleed easily.

That's interesting. Im going to have to look through some junkyard cars.

I could easily T one into the lower radiator hose, but that's right where the thermostat is. Any reason why that would be a bad idea?

if you are going to go to the effort to put a T in the hose to run a cap.. why not just run a saab expansion tank?

I will pull the one out of car today or tomarrow (runnin 5 hours of sleep right now, so I might not make it to the garage and it is your's (With cap) for the price of shipping

mad_machine wrote: if you are going to go to the effort to put a T in the hose to run a cap.. why not just run a saab expansion tank? I will pull the one out of car today or tomarrow (runnin 5 hours of sleep right now, so I might not make it to the garage and it is your's (With cap) for the price of shipping

I appreciate the offer, but someone local will just give me one. Can I T into the lower radiator hose on my Subaru motor? The Saab tank T's into the water pump inlet. I would be T'ing into the hose between the radiator and the thermostat.

as noted before GMs from 20 years ago-forward have expansion tanks, my XR4 from 1986 has an expansion tank, they are nice because they are self bleeding systems. My Fiat from 72 however has an overflow tank and is a PITA to bleed...

Raze wrote: as noted before GMs from 20 years ago-forward have expansion tanks, my XR4 from 1986 has an expansion tank, they are nice because they are self bleeding systems. My Fiat from 72 however has an overflow tank and is a PITA to bleed...

I'm still fuzzy on how they actually maintain a given pressure. The whole system is pressurized. It must bleed something off to maintain a specific pressure. The cap has no overflow tubes on the Saab as far as I can tell. So does the cap just bleed off air?

It just vents to the atmosphere. See the check valves in the bottom?

Yep. Excellent. Thanks.

So basically instead of having a nipple for 'stuff' to bleed out, it just bleeds off at the cap. In the case of a properly filled expansion tank, this would be air.

I will gladly run an expansion tank if I can T into the lower radiator hose which goes directly to the thermostat. I can't see why this wouldn't work.

My 2002 Liberty (KJ) had that system., also my 2011 Fiesta has one.. Look at it as a remote header tank for the radiator.

OK so in that case, it wouldn't really matter at all where it was T'd into!

Taiden wrote: OK so in that case, it wouldn't really matter at all where it was T'd into!

It does matter where you place the T. The T should be in an area of pressure lower than the top of the radiator. Most return hoses are connected near the inlet for the water pump.

So now I don't really know. Because the place that's easy to T into is between the lower radiator outlet and the thermostat housing. I'm imagining that the restriction of the thermostat would cause a build up of pressure at that area.